Resolves Gradle deprecation and language version warnings
```gradle
Java compiler version 21 has deprecated support for compiling with source/target version 8.
```
The `java.toolchain.languageVersion` syntax is a superset that covers the previous syntax `android.compileOptions`
Removes a number of unused functions from the file, renames remaining
functions to match our builder script prefix patterns, removes unused
imports from various build scripts (e.g. Android).
Fixes: #14453
Clarifies the confusing builder.inc.sh / build-utils.sh distinction by
giving the scripts more appropriate names. Most build scripts should use
builder-full.inc.sh; some helper scripts can use builder-basic.inc.sh.
Documented in resources/build/README.md.
Renames:
* resources/build/builder.inc.sh to resources/build/builder-full.inc.sh
* resources/build/build-utils.sh to resources/build/builder-basic.inc.sh
Other changes:
* Moves Android-specific functions out of builder-basic.inc.sh and into
android/build.sh.
* Renames functions in builder-basic.inc.sh
More functions may be moved from builder-basic.inc.sh into utils.inc.sh
or other scripts in the future.
Fixes: #14065
Build-bot: build all
Test-bot: skip
Adds documentation for various functions, removes unused functions (a
couple of simple, very lightly used functions were unDRYed; these could
go back the other way across all shell scripts if necessary). TODO items
noted for follow-up refactoring.
Fixes: #14275
Relates-to: #14269
Build-bot: build all
Test-bot: skip
Previously the builder scripts defined a readonly `VERSION` environment
variable for the Keyman version. That caused problems when another
(external) script tried to define a `VERSION` variable. We encountered
this problem when trying to move the TC build steps of a configuration
into a single script (#13399) when we tried to source `~/.nvm/nvm.sh`.
This change uses a Keyman specific prefix for the version variables and
renames `VERSION` → `KEYMAN_VERSION` etc. Unfortunately these variables
are used in a lot of places, so this turned out to be a bit of a yak
shave.
Test-bot: skip
From the days of stable-12.0 when the FirstVoices for Android app was configured in the main repo, the SystemKeyboard initialized with
3148aa7b8c/oem/firstvoices/android/app/src/main/java/com/firstvoices/keyboards/SystemKeyboard.java (L111-L113)
In the scenario FV for Android installed and enabled as the system keyboard (from the Android settings menu instead of launching the FV app), the initial usage as a system keyboard would have an uninitialized keyboard key (language and keyboard ID pairing to determine the current keyboard).
Fixes: FV-ANDROID-5G
Fixes: FV-ANDROID-7N
Note: there is a bit of potential confusion about the difference between
/resources/builder.inc.sh (the full implementation for builder scripts),
and /resources/build/builder.inc.sh (the source script that builder
scripts should always use).
This allows us to make assumptions that will always be true for builder
scripts that may not be true for other scripts, such as setting base
folder.
