* Bump SDK revision to 9123335
* Remove no longer used platforms;android-21
* Do not remove "unused parts" from SDK (which are actually used)
* Make it possible to use alternative JAVA_HOME via TERMUX_JAVA_HOME
And keep ndk-patches in 23c/ subdirectory. Run
termux_step_setup_toolchain_23c only if TERMUX_NDK_VERSION equals 23c.
This is a step towards having the possibility to use different NDK
versions. Using a different NDK version than the one termux
officially supports should *really* not be done except for
testing/debug/development reasons, or if it is strictly necessary to
be able to compile a program (for example for packages that need a
fortran compiler, which at the moment is only supported with old
gcc-using NDKs).
Dockerfile setup script doesn't know exactly which ndk/sdk version we
are using, so can't cd to the right folder now that the version is
part of the folder name.
