Precalculate the required number of characters for path shortening

It does not make sense for aapt2 to calculate the optimal number of
chars in the shortened path when it only depends on the number of
resources rather than anything apk specific. This cl precalculates the
number of chars and uses if conditions instead.

Test: manual testing + aapt2_tests still passes.
Change-Id: If2dc86952a6b6e01ce02dc42754c037d07d57325