StringBlock incremental hardening default values

When a string cannot be retrieved from a StringBlock due to pages of
the StringPool missing, rather than throwing an
IndexOutOfBoundsException, compute a default value rather that
abides by the API contacts of the functions that use the StringBlock.

IndexOutOfBoundsExceptions are not really documented as a possible
exception that can be thrown from APIs that use StringBlock because
it indicates that the binary resource table was built incorrectly.

Returning default values is much less likely to break applications
that fetch the resources of incrementally installed applications than
throwing IndexOutOfBoundsExceptions where they would normally not be
expected.

Bug: 188174746
Test: atest ResourcesHardeningTest
Change-Id: I58fe754fb446fefc031ddba19e290830fdd6d015