Fix handling of non-integer ANDROID_PLATFORM

ANDROID_PLATFORM is not always an integer; it can also be a value like
"36.1". Handle such cases properly.

This fixes the following error:

    server/build_without_gradle.sh: line 89:
    [[: 36.1: syntax error: invalid arithmetic operator (error token is ".1")

PR #6408 <https://github.com/Genymobile/scrcpy/pull/6408>

Signed-off-by: Romain Vimont <[email protected]>