Change kMultiDexSeparator from ':' to '!'

The ':' separator is commonly used to separate class path elements. That
means that we cannot easily encode multidex location in a classpath
without complicating the parsing logic unnecessarily (e.g. when encoding
classpaths in the oat file).

For easy parsing and understanding kMultiDexSeparator and
kClassPathSeparator should have different values. ':' is a wide spread
classpath separator so this CL changes the value of kMultiDexSeparator to
'!' which is also commonly used to denote an object inside a given
container.

Test: m test-art-host
Bug: 38138251
Change-Id: I30995c553d9131478c6c071b27327df6d2de06a7